F2 Red-eyed male Red-eyed male Red-eyed female White-eyed female
XR XR XW XR XR YW XW YW
Homozygous. Heterozygous. Heterozygous. Homozygous.

White-eyed male Red-eyed female
XW XW x XR YW

F1 Red-eyed male White-eyed female
XW XR XW YW

F2 White-eyed male Red-eyed male White-eyed female Red-eyed female
XW XW XR XW XW YW XR YW
Homozygous. Heterozygous. Homozygous. Heterozygous.

It must be explained that according to this theory the normal male is always heterozygous, because the Y chromosome never carries any other factor except that for sex; it is thus of no more importance than the absence of an X chromosome which occurs in those cases where the male has one sex-chromosome and the female two. According to the researches of von Winiwarter [Footnote: 'Spermatogénèse humaine,' Arch. de Biol., xxvii., 1912.] on spermatogenesis in man, the latter is actually the case in the human species. This investigator found that there were 48 chromosomes in the female cell, 47 in the male; after the reduction divisions the unfertilised ova had 24 chromosomes, half the spermatids 24 and half 23, so that sex is determined in man by the spermatozoon.

Morgan believes that the heredity of haemophilia (the constitutional defect which prevents the spontaneous cessation of bleeding) follows the same scheme, and also at least some forms of stationary night-blindness— that is, the inability to see in twilight.

We may mention a few other in animals, referring the reader for a fuller account to the works cited. One example in the barred character of the feathers in the breed of fowls called Plymouth Rock. In this the female is heterozygous for sex as in Abraxas grossulariata, and the barred character is sex-linked. When a barred hen is crossed with an unbarred cock all the male offspring are barred, all the females plain. On the other hand, if a barred cock is crossed with an unbarred hen, the barred character appears in all the offspring, both and females. The female thus transmits the character only to her sons. If we represent the barred character by B, and its absence by b, we can represent the heredity as follows:—

BARRED FEMALE WITH UNBARRED MALE

B female b male X b male b male

Bb male bb female